Polysilicon thin films have been used previously in order to simplify the manufacturing processes and reduce the manufacturing costs of thin-film transistors and thin-film solar cells. Although these polysilicon thin films can be obtained, it is not possible to use inexpensive glass substrates (usual softening point not more than 650.degree. C.) since with the prior art a temperature of not less than 650.degree. C. is required. On the other hand, with glass substrates having a high softening point, there is a problem of diffusion of impurities in the substrate into the polysilicon, and a further problem is that due to the difference in thermal expansion coefficient between the polysilicon and the glass substrate, it is difficult to deposit polysilicon onto a glass substrate.
Moreover, in order to minimize carrier recombination in the polysilicon thin film, the crystal grain diameter (below, grain size) needs to be controlled. Since the grain size of the polysilicon thin film is influenced by the substrate temperature, the grain size of polysilicon thin films is generally controlled by varying the substrate temperature. At that time, the substrate temperature is varied between 600.degree. C. and 1100.degree. C. Accordingly, for the above reason, it is not possible to control the grain size of polysilicon formed on inexpensive glass substrates. For a similar reason, it is not possible to control the grain size of polysilicon thin films formed on metal substrates.
On the other hand, in a recently proposed annealing method in which a polysilicon film is obtained by annealing an amorphous silicon film using an excimer laser, it is necessary to move the substrate or the optical system because of the small size of the laser spot. A problem here is that it is difficult to obtain a uniform polysilicon film over a large area, since either the crystallization at the boundary region is uneven or there is a remnant of the amorphous part.
Furthermore, another disadvantage concerning the control of the grain size of the polysilicon film is that, since the grain size is basically determined by the substrate temperature, it is difficult to form a product having a large grain diameter.
The present invention has been realized in consideration of these kinds of problems inherent in the prior art, and accordingly, the main object of this invention is to provide a polysilicon thin film which can be formed on an inexpensive glass substrate or metal substrate, and moreover, whose grain size can be controlled, and a production method therefor. A further object of the present invention is to provide a substrate for forming a thin polysilicon film and a production method therefor, as well as a radical source used for forming a polysilicon thin film.